The invention relates to a method for the dynamic monitoring of a first lambda probe arranged in an exhaust-gas duct of an internal combustion engine upstream of an exhaust-gas purification system, a period of an output signal of the first lambda probe being determined in a controller of the internal combustion engine, and a lambda correction being determined from an output signal of a second lambda probe connected downstream of the exhaust-gas purification system.
The invention also relates to a method for detecting a defect upstream of a second lambda probe arranged in an exhaust-gas duct of an internal combustion engine and connected downstream of an exhaust system, a lambda correction being determined in a controller of the internal combustion engine from an output signal of the second lambda probe.
The invention also relates to a device for the dynamic monitoring of a first lambda probe arranged in an exhaust-gas duct of an internal combustion engine upstream of an exhaust-gas purification system, a circuit arrangement or a program sequence being provided in a controller of the internal combustion engine, by means of which circuit arrangement or program sequence a period of an output signal of the first lambda probe can be determined, and a second lambda probe for determining a lambda correction being connected downstream of the exhaust-gas purification system.
Lambda probes are used in the exhaust tract of internal combustion engines to measure the oxygen content of the exhaust gas in order to control the composition of the air/fuel mixture of the internal combustion engine. Lambda probes may be designed as step probes, the output signal of which falls abruptly from 0.9 V to 0.1 V in the event of a change of the lambda value from 0.995 to 1.005. The output signal of the lambda probe is supplied to an engine controller which controls the metering of the fuel in such a way that, temporally on average, a lambda value of lambda=1 is adhered to, at which the catalytic converters arranged in the exhaust tract provide their optimum purification action. If a lambda probe ages, this can lead to a delayed reaction of the output signal to lambda changes, a so-called impairment of dynamics. In this way, the composition of the exhaust gas may intermittently deviate from a value suitable for an optimum purification action of the catalytic converters. Legal regulations therefore stipulate that the aging of the lambda probe must be monitored with regard to an impairment of its dynamics. A slowing of the reaction of the lambda probe can be detected from a lengthening of the period of the lambda regulation, which can therefore be taken into consideration as a criterion for aging.
If the delay of the reaction of the lambda probe is asymmetrical with regard to rich-lean and lean-rich lambda changes, this can lead to a change in the mean lambda value controlled by the engine controller, as a result of which the purification action of the catalytic converters is particularly disadvantageously reduced. This may be observable even in the case of a delay which cannot be detected from period-based monitoring.
If a leak occurs in the exhaust tract upstream of the second lambda probe, air can be sucked into the exhaust-gas duct and, by means of its oxygen content, increase the lambda value determined by the second lambda probe. As a result, an undesirably rich mixture is supplied to the internal combustion engine.